1. Field of the Invention
The present invention relates to an image projector for projecting an image to a projection plane. More particularly, the invention relates to, especially, an optical system of a small-sized image projector for projecting an image by two-dimensionally deflecting and scanning light. More specifically, the invention relates to a so-called optical engine part of a small size in, for example, a pocket projector, a data projector, a rear projection television, or the like.
2. Description of the Background Art
Various small-sized image projectors for two-dimensionally deflecting and scanning light from a light source and projecting an image are proposed (for example, Japanese Patent Application Laid-Open No. 2006-178346 and SID 06 DIGEST, pp. 2015-2017, 73.3). As means for two-dimensionally deflecting light (light scanning means), a galvanometer mirror or a so-called MEMS (Micro Electro Mechanical Systems) mirror is used.
In such an image projector, to project a dynamic image on a large screen, the galvanometer mirror or MEMS mirror has to be driven at high speed and with large amplitude (mechanical deflection angle). For example, a system of drive using resonance (resonant drive) is employed.
However, it is known that, in the case of employing the resonant drive, the scanning speed largely drops at both ends of an amplitude, that is, at the maximum and minimum values of the amplitude, in other words, in peripheral parts of an image. It is also known as a general problem that, due to such a phenomenon, brightness in peripheral parts of a screen becomes higher than that in the center of the screen and, when the modulation speed of the light source is constant, a difference occurs between the resolution in the center of the screen and the resolution in a peripheral part of the screen. It is also known as another general problem that, when a projection method of projecting an image onto a projection plane (for example, a screen) from an oblique direction is considered, an image projected onto the projection plane (projected image) is distorted in a trapezoidal shape.
To address such problems, a method is considered in which a correction is excellently performed by using an optical system so as to achieve constant speed of a light flux emitted to a projection plane (for example, a screen) in order to suppress variations in brightness and resolution by employing deflection scanning means using a mirror of a resonant drive system. An optical scanning device has been proposed, that corrects fluctuations in scan speed and a distortion in an image by using a projection optical system provided with the so-called f-arcsine characteristic and including a combination of two mirrors and a single refractive lens or a combination of three mirrors (for example, Japanese Patent Application Laid-Open No. 2006-178346). A technique of correcting fluctuations in scan speed and a distortion in an image by using an image process has been also proposed (for example, SID 06 DIGEST, pp. 2015-2017, 73.3).
In the technique of Japanese Patent Application Laid-Open No. 2006-178346, fluctuations in scan speed and a distortion in a projected image are corrected in an apparatus using an MEMS mirror for two-dimensionally deflecting and scanning light. Problems caused by the driving method and the projecting method of the MEMS mirror are corrected by an optical system (projecting optical system) for projecting light deflected by the MEMS mirror onto a projection plane. Consequently, it causes enlargement in the size of the projecting optical system and the image projector. That is, when resonant drive in the direction along the horizontal scan line (the lateral direction) and linear drive in the vertical direction are corrected only by the projecting optical system while using the MEMS mirror, the projecting optical system is complicated, and the size of the projecting optical system is enlarged. Therefore, although miniaturization is originally aimed, the apparatus is enlarged, and it can be said that needs are not responded properly. When problems caused by the driving method and the projecting method of the MEMS mirror are corrected by using an image process like the technique of SID 06 DIGEST, pp. 2015-2017, 73.3, the angle used for deflection of light and projection decreases in the mechanical deflection angle of the MEMS mirror. Therefore, deterioration in the so-called duty causes decrease in brightness of an image, that is, deterioration in picture quality.